This invention relates generally to polyphase, multi-circuit dynamo electric machine winding patterns and more particularly to winding patterns for four pole, three phase, dynamo electric machine with three parallel connected circuits per phase.
A conventional turbine-driven polyphase generator of large size comprises a stationary armature element and a cooperating rotatable field element, and may be operative to generate three-phase electric power, whereby the field element, if of four-pole construction, is rotated at 1800 r.p.m. for a frequency of 60 hertz, or is rotated at 1500 r.p.m. for a frequency of 50 hertz. In the generator, the armature element or core is formed of magnetizable material .and is normally provided with an elongated cylindrical opening therethrough that is bounded by a number of angularly spaced-apart winding slots, in which a composite three-phase armature winding is carried, the armature winding including three individual phases that are frequently connected in Y or star relation to supply the load. The field element is also formed of magnetizable material and is normally of the elongated cylindrical-rotor type and carries a field winding, the field element being arranged in the armature opening and being operative upon rotation to induce phase voltages in the respective phases of the armature winding.
Ordinarily, each phase of the armature winding includes a number of phase belts each provided with a plurality of series connected coils each having two coil sides, the coil sides being distributed in the winding slots of the armature element; the armature winding is arranged in two layers so that each winding slot receives two of the coil sides respectively disposed in the top and in the bottom thereof; and each of the coil sides includes an insulating casing enclosing the conductors thereof for the purpose of electrically insulating the coil side from the contiguous coil side and from the armature element
In directly cooled machines, each of the winding slots also accommodates facility for cooling the two contained coil sides, which cooling facility ordinarily includes a duct arrangement through which a current of air or other gas, or stream of liquid is forced; whereby each of the slots must accommodate two of the coil sides, as well as the cooling facility therefor; and each of the coil sides includes the conductors thereof and the enclosing insulating casing.
Now in the design of polyphase generators, there is an ever-increasing demand for greater power ratings; whereby the competition for space in the armature between the magnetic material thereof and the winding slots therein, and among the several elements arranged in each winding slot thereof, presents design problems of ever-increasing complexity.
In order to obtain a maximum power output from a polyphase generator structure of given physical dimensions, it has been proposed in U.S. Pat. No. 2,778,963, granted on Jan. 22, 1957 to Rudolph Habermann, Jr., that each phase of the armature winding be provided with three parallel circuits arranged in a pair of series connected sections respectively disposed in a pair of oppositely poled phase belts, wherein the coil sides of the three parallel circuits are arranged in a predetermined sequence in the slots in the phase belts in order to minimize voltage and phase unbalance among the three individual circuits of the phase mentioned.
U.S. Pat. No. 3,201,627 issued to Dean B. Harrington on Aug. 17, 1965 describes a number of winding patterns for four-pole, three-phase armature windings having three parallel-connected circuits per phase. The patterns illustrated in that patent for a 72 slot core structure give a high degree of balance among the three parallel-connected circuits, both with respect to magnitude of voltage unbalance and quadrature unbalance (phase angle displacement) between a given circuit and the phase.
Although the winding patterns disclosed in the Harrington patent are quite suitable from an electrical standpoint, in that there is only negligible increased relative heating due to the circulating currents between the three parallel-connected circuits, there are some mechanical complexities involved in use of the patterns. U.S. Pat. No. 3,476,964 issued Nov. 4, 1969 to David M. Willyoung disclosed an improved four-pole, three-phase armature winding having three parallel-connected circuits per phase, which greatly reduced the mechanical complexities of the aforesaid Harrington patent without significantly affecting the circulating currents between the three parallel-connected circuits. The increased relative heating due to such circulating currents becomes less significant when internal direct cooling of the winding is employed and it overshadowed by the higher power ratings which may be possible by improved winding patterns, especially in the case of generators used in large nuclear or fossil power generation plants.
In some generator designs, certain economies and technical benefits are achieved by reducing the number of armature slots, particularly in directly-cooled generators of very large ratings. If it is desired to reduce armature bar forces by employing multi-circuit windings, it is generally more difficult to find suitable patterns as the number of circuits increase and the number of slots decrease. To illustrate, the number of slots in a three phase winding with three parallel connected circuits per phase must be divisible by nine. For example, reducing the number of slots from the 72 slot pattern shown in the aforesaid Harrington and Willyoung patents gives the possibility of 63, 54, 45 . . . etc. slots. Heretofore, efforts to reduce the number of slots without creating substantial circulating currents among the three circuits have been unsuccessful. Furthermore reducing the number of slots without reducing the number of phases or the number of parallel-connected circuits per phase gives the possibility of "fractional slot" windings, which have generally been avoided in the past.
Accordingly, one object of the present invention is to provide improved winding patterns for a three phase, four pole, three circuit dynamo electric machine with acceptably small circulating currents between circuits.
Still another object is to provide an improved three-phase, four-pole, 63 slot armature winding having three parallel-connected circuits per phase, and utilizing a fractional slot winding with suitably low electrical unbalance between circuits so that circulating currents are inconsequential.